Mammary prostheses which comprise flexible shells of a silicone polymer filled with a silicone gel are disclosed in U.S. Pat. No. 3,293,663 and U.S. Pat. No. 3,665,520. Silicone gel is generally considered to be an ideal material for surgical implants as it has a natural tissue like resiliency it does not cause tissue reaction, and it has approximately the same specific weight as normal mammary tissue. However, recently it has been determined that in some instances the fact that silicone gel has a specific weight approximately the same as normal human mammary tissue is not an advantage and that a lighter material might be more suitable. In an implanted mammary prosthesis of significant size a lighter weight material is preferred because the weight of a mammary prosthesis is carried to a large extent solely by the skin capsule in which it is implanted whereas, the weight of normal mammary tissue is distributed fairly evenly through its attachment to the fascia and via the supporting Cooper's ligaments. A lighter material is often desirable in an external mammary prosthesis because the entire weight of such a prosthesis is concentrated on the shoulder straps of the brassiere. Previous attempts to develope a lighter weight silicone gel have been unsuccessful as they have resulted in prostheses which either lacked the resiliency of normal tissue or else contained materials that were palpable.